Computing systems generally use one or more display monitors to provide a visual input/output capability. While the structures of computer display monitors varies, all generally include a cathode ray tube (CRT) having an evacuated envelope usually made of high-strength glass. The envelope defines a generally flat or slightly curved faceplate together with a funnel shaped bell and extending neck. The interior side of the faceplate supports a phosphor display screen. In monochrome displays, a single electron gun is supported within the CRT neck and is directed toward the phosphor screen. The electron gun produces a beam of electrons which are directed toward the faceplate striking the phosphor screen and causing visible light to be emitted therefrom. In color display systems, a plurality of electron guns are used together with a phosphor screen which supports plural areas of phosphors having differing color light emitting characteristics. A shadow mask or similar structure is interposed between the electron guns and the phosphor screen to cause each of the electron guns to stimulate an associated type of colored light emitting phosphor.
Whether the display system is monochrome or color, the electrons emanating from the electron gun or guns form a CRT beam which is scanned in both the horizontal and vertical directions across the faceplate to form a raster. In most instances, the horizontal scan system is operative at a higher frequency than the vertical scan system. Thus, the horizontal scan moves the electron beam rapidly from side to side across the faceplate while the vertical scan system causes the successive horizontal scans to be moved progressively from top to bottom to complete a display frame and form the raster.
In the majority of the presently used display systems, electron beam scanning is accomplished by electromagnetic deflection of the CRT beam. A deflection yoke is supported upon the CRT envelope between the electron guns and the faceplate. The deflection yoke supports a plurality of deflection coils which are coupled to the horizontal and vertical scan systems. Horizontal and vertical scan signals provided by the respective scan systems are coupled to the windings of the deflection yoke to produce corresponding electromagnetic fields which bend the electron beam and thereby direct it to the desired portion of the CRT faceplate. Both the horizontal and vertical scan signals include longer duration sloped scan portions followed by shorter duration high amplitude retrace portions. The latter are utilized at the completion of each respective scan interval to return the electron beam to its starting position. In addition, the retrace portion of the horizontal scan signal is used to develop the high voltage necessary to accelerate the electron beam toward the CRT faceplate.
The character of the image displayed in a CRT display system results from variation or modulation of the intensity of the scanned CRT electron beam. This intensity modulation must be properly timed or synchronized to the horizontal and vertical rate scanning of the raster. Thus, as the electron beam is scanned across the faceplate to form a raster, the desired portions of the faceplate are illuminated by synchronized modulation of the electron beam to provide the desired image.
Most display monitors operate at constant scan frequencies and use a single image size format. Great care is taken to establish and maintain constant scan frequencies and precise relatively constant image size. However, it has been found desirable in certain applications to operate the monitor at one of several different scan frequencies. It is also advantageous to include the capability for expansion or enlargement of the displayed image to produce an enhanced more dramatic effect.
Image expansion is typically accomplished by deliberately inducing an "overscan" condition within the display monitor. Most monitors achieve overscan by simultaneous increase of the vertical and horizontal deflection currents driving the deflection yoke. In effect, the raster is scanned beyond the border or outer edge of the CRT faceplate. Systems having overscan capability are subject, however, to additional problems not encountered by single display size monitors. For example, the precise adjustment and maintenance of the multiple size formats is often difficult to achieve. Frequently, the size changing mechanism introduces inaccuracies in the size control systems tending to degrade overall performance of the display monitor.
Operation of the display monitor at a variety of scan frequencies usually creates undesired amplitude changes in the monitor deflection systems.
In vertical deflection systems, the amplitude of scan signal and scan frequency are interactively related due primarily to the use of saw forming capacitor networks to generate the basic scan signals. Such systems use switching type oscillators to repeatedly charge and discharge a saw forming capacitor at constant current rates. As a result, attempts to change frequency disturb amplitude and vice versa.
Despite the attendant problems, multiple frequency display monitors having expandable image capability are advantageous to the user. There remains, therefore, a need in the art for an inexpensive, efficient multiple frequency image expanding display system operable upon the vertical deflection systems of monitors which effectively provides for independent control of image size in a multiple frequency operating mode.
Accordingly, it is a general object of the present invention to provide an improved display monitor. It is a more particular object of the present invention to provide an improved display monitor having image expanding capability which substantially maintains image size stability in each mode of operation. It is a still more particular object of the present invention to provide a vertical scan system for use in a display monitor which is operable in an image expanded mode and which may be used at a variety of scan frequencies.